This invention relates generally to improvements in motor control systems and, more particularly, to a new and improved digital servo motor control system for a d.c. motor to render such a motor suitable for use in a syringe pump for parenteral administration (referred to herein as "intravenous administration" or "IV administration") of medical fluids over an extremely wide range of fluid flow rates, whereby high speed d.c. motor and low speed stepping motor performance is achieved, with stepping motor precision, yet in a more compact, lightweight, lower cost arrangement requiring less power and fewer electrical components.
The usual medical procedure for the gradual IV administration of fluids into the human body, such as fluid replacement, liquid nutrients, blood or plasma, makes use of apparatus which is commonly referred to in the medical arts as an intravenous solution administration set. Such a set typically is a disposable plastic product, and comprises a drop chamber adapted to be connected to a fluid source, a length of tubing extending from the chamber to the patient and a valve mechanism, such as a roller clamp on the tubing.
The drip chamber of the IV administration set serves a dual function of allowing a nurse or other attendant to observe the rate at which the fluid drips out of the fluid source and also creates a reservoir for the fluid at the lower end of the drip chamber to insure that no air enters the main feeding tube leading to the patient.
While observation of the rate of drop flow via the drip chamber is a simple way of controlling the amount of fluid fed to a patient over a period of time, its ultimate effectiveness requires that a relatively constant vigil be maintained on the drop flow, lest it cease entirely due to exhaustion of the fluid supply or vary unacceptably from the set rate.
In addition to the aforedescribed difficulties, the IV administration of medical fluids by gravity induced hydrostatic pressure infusion of the liquid from a fluid source suspended above a patient, may be susceptible to fluid flow rate variations due to changes in the fluid level in the bottle, changes in temperature, changes in the venous or arterial pressure of the patient, patient movement, and drift in the effective setting of the roller clamp or other valve mechanism pinching the feeding tube. Moreover, there are a number of situations, such as in intensive care, cardiac and pediatric patients, or where rather critical drugs are being administered, where the desired drop flow rate must be capable of rather precise selection and must not drift beyond certain prescribed limits in spite of varying load conditions.
In view of the foregoing, a number of electrical monitoring systems, drop flow controllers and infusion pumps have been developed in recent years to infuse medical fluids into patients at precisely regulated fluid flow rates. However, while such devices have generally served their purpose, there is a continuing need for improvement in accuracy and precision of adjustment over a wide range of selected flow rates. In this regard, difficulties have been experienced in connection with establishing and maintaining such accurate fluid flow rates at the extreme ends of the operating range, i.e., at very high flow rates and very low flow rates over a wide flow rate range such as 1,000 to 1, and there has been a desire to reduce the size, weight and complexity of the infusion devices without any reduction in accuracy and reliability.
Syringe pumps have been developed and have become popular in the IV administration of fluids into the human body, such syringe pumps typically embodying a motor driving a piston within a syringe to expel fluid from the syringe at a controlled rate through a length of tubing and into the patient. In the past, such syringe pumps have commonly used relatively bulky and expensive stepping motor drives which require holding current circuits, ramp up/ramp down circuits and special mechanical coupling techniques to obtain wide operating speed ranges. In addition, the torque characteristics of typical stepping motors tend to vary with speed and introduce additional difficult design problems, and stepping motors typically cannot provide adequate high speed performance.
While d.c. motors offer lower power consumption and higher speed performance, in a more compact, lightweight, structural arrangement, such d.c. motors lack the low speed performance and precision of stepping motors. In this regard, conventional d.c. motors tend to overshoot and undershoot with changing loads and speeds rather than moving in precisely commanded increments of motion. In addition, d.c. motor drive circuits may require digital-to-analog conversion hardware and torque control circuitry, which increases power loss and circuit complexity. Moreover, temperature and voltage variations can decrease speed control accuracy with conventional d.c. motors.
Hence, those concerned with the development and use of motor control and drive systems of the type suitable for use in IV fluid administration systems, and particularly those concerned with the design of IV syringe pumps, have long recognized the need for improved, relatively simple, economical, compact, reliable, lightweight, stable and accurate devices, including improvements in motor control systems for effectively accomplishing a wider motor speed control range and a concomitant wide range of precisely delivered fluid flow rates, in order to obviate the aforedescribed difficulties. The present invention clearly fulfills this need.